In a ring laser gyroscope (RLG), to obtain accurate inertial rotation information, a constant pathlength has to be maintained. To do so, U.S. Pat. No. 4,320,094 teaches the use of an electronic circuit for calculating the respective power of the circulating light beams in the cavity of an RLG. The power thus calculated is then used to modulate a piezoelectrically activated movable mirror assembly for maintaining the pathlength constant. However, due to the fact that the signals used for calculating the power are the same signals used for generating the RLG fringe pattern, noises are inherently added into the calculated power.